1. Field of the Invention
The present invention relates to an electric cable with increased flexibility and peeling-off properties.
Moreover, the present invention relates to an electric cable with increased intelligibility of the marked indicia thereon.
In particular, the present invention relates to an electric cable for power transmission at low voltage, preferably said electric cable being suitable for building wiring.
Furthermore, the present invention relates to a manufacturing process of said electric cable.
In the present description, the expression “low voltage” means a voltage of less than about 1 kV, the expression “medium voltage” means a voltage of between about 1 kV and about 30 kV, the expression “high voltage” means a voltage of between about 30 kV and about 220 kV, while the expression “extra high voltage” means a voltage of greater than about 220 kV.
2. Description of the Related Art
Cables for power transmission at low voltage are generally provided with a metallic conductor which is surrounded by an insulating coating adhering to said metallic conductor.
In the present description the expression “cable core” indicates a structure comprising at least one conductor and a respective electric insulating coating arranged in a position radially external to said conductor.
For the purposes of the present description, the expression “unipolar cable” means a cable provided with a single core as defined above, while the expression “multipolar cable” means a cable provided with at least one pair of said cores. In greater detail, when the multipolar cable has a number of cores equal to two, said cable is technically defined as being a “bipolar cable”, if there are three cores, said cable is known as a “tripolar cable”, and so on.
In a position radially external to said insulating coating, cables for power transmission at low voltage can be provided with an outer polymeric sheath having the function of mechanically protecting the cables from the external environment, e.g. from any impact and/or abrasion that might lead to cable cracks or ruptures formation. In a multipolar type configuration, in the case said outer protective polymeric sheath is present, the multipolar cable is provided with a common sheath surrounding the cable cores as defined above.
Document. U.S. Pat. No. 4,789,589 discloses an electric cable provided with a double layer insulating coating arranged upon a conductive element, said double layer comprising an inner layer of a polyolefin compound and of a cellular construction, and an outer layer of a given maximum thickness and of a non-cured and non-curable polyolefin base compound having a solid (i.e. non-expanded) construction, said base compound including a material compatible with the polyolefin of the inner layer. Preferably, the polyolefin of the outer layer is polyvinyl chloride. Document U.S. Pat. No. 4,789,589 refers to the problem of separation of the thin uncured outer layer from the inner layer during the manufacturing process and/or during installation of the cable. The Applicant observed that such a solution can not correctly work since the inner insulating layer, due to its expanded state, presents discontinuities (i.e., voids within the polymeric material, said voids being filled with air or gas) in the space surrounding the conductor, i.e. in the space where the electrical field is the most relevant.
Data communication cables provided with a double layer insulating coating comprising an inner layer of an expanded polyolefin compound and an outer layer of a non-expanded polyolefin base compound are described in documents CA-952,991 and U.S. Pat. No. 5,841,072.
Document JP 90-35544 discloses an electric low voltage distribution cable comprising a pair of twisted insulated conductors between which a foaming material is arranged. A sheathing material is also provided for covering the twisted cable cores. The foaming and the insulating material can be polyvinyl chloride.
Document U.S. Pat. No. 3,013,109 relates to non-metallic sheathed cables for building wiring comprising a protective sheath made of expanded cellular organic material. According to said document, the insulation is made of a dense solid material (e.g. a dense semi-rigid polyvinyl chloride) while the outer protective sheath is composed essentially of a tough, flexible resinous plastic material such as polyvinyl chloride in expanded cellular form. Furthermore, document U.S. Pat. No. 3,013,109 states that the outer protective sheath is distinct from the insulation and is slippable or slidable thereon with the result that the cable can be readily bent without distortion in the plane of the conductors and, after being deliberately bent at a selected angle, the cable maintains the bent shape indefinitely.
Document JP 11-203941 discloses a method for manufacturing a cable provided with an insulating coating obtained from a resin composition mainly comprising vinyl chloride resin, and with an expanded sheath layer obtained from a composition mainly comprising vinyl chloride resin into which a foaming agent has been compounded.
Document WO 98/52197, in the name of the Applicant, discloses the use of a layer made of expanded polymeric material of a suitable thickness to be applied in a position radially external to the aforesaid cable core. According to said document, the expanded layer confers the cable high resistance to accidental impacts which might be suffered by the latter during the steps of cable transport or laying. Said impacts are very dangerous for the cable since they can cause considerable damage to the cable structure (for example deformation of the insulating layer, detachment of the cable layers) determining, for example, changes in the electrical gradient of the insulating layer with a consequent reduction in its insulating capacity. Said expanded layer is preferably located in a position immediately below the outer polymeric sheath of the cable and, being able of endowing the cable with high impact strength, it makes possible to eliminate any traditional, generally metallic, protective armours. In order to confer the cable the desired impact resistance, the expanded layer is obtained from a polymeric material which, before expansion, has a flexural modulus at room temperature (measured according to ASTM Standard D790) of at least 200 MPa.
Document U.S. Pat. No. 3,936,591 discloses a non metallic-sheathed cable for building wiring comprising a wall of expanded polyvinyl chloride insulation surrounding each cable conductor and a thin-walled tubular jacket of polyvinyl chloride surrounding the totality of insulated conductors, said jacket conferring mechanical protection to the non metallic-sheathed cable.
Generally, a cable for building wiring is installed within the walls of a building and the installation process requires that the cable passes through walls restrictions or, more frequently, that the cable is pulled through conduits, wherein the cable is permanently confined.
In order to be correctly installed with simple and quick operations, a cable for building wiring needs to be particularly flexible so that it can be inserted into the wall passages and/or wall conduits and follow the bends of the installation path without being damaged.
During customer installation, due to the tortuosity of the installation path and to friction during the pulling operation, the cables for building wiring are generally subjected to tearing or scraping against rough edges and/or surfaces.
The Applicant has perceived that an increased flexibility of an electric cable for building wiring can allow to reduce the damages caused by said tearing or scraping actions.
Furthermore, a very important aspect which is required to be satisfied by a cable for building wiring is a simple and quick peeling-off of the cable. In the present description the term “peeling-off of a cable” is used to indicate the removal of all the cable layers which are radially external to the conductor so that it results uncoated and can be electrically connected to a conductor of a further cable or to an electrical apparatus.
The peeling-off property of a cable for building wiring is a widely felt request of the market since the peeling-off of a cable is an operation which is manually performed by the technical staff. For this reason, said operation is required to be easy and quick to be performed by the operator, taking also into account that it is frequently carried out in narrow spaces and rather uncomfortable conditions.